Update style_guide c++ version.

The CMake file states c++17, Chromium is at c++17. This CL updates the
style guide documentation to specify c++17 as required for Tint.

Change-Id: I4dd8e9aa69de6f32a59ced2c5b861c83adf2edfe
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/85320
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@chromium.org>
Commit-Queue: Ben Clayton <bclayton@google.com>
diff --git a/docs/tint/style_guide.md b/docs/tint/style_guide.md
index fce4441..52b08c5 100644
--- a/docs/tint/style_guide.md
+++ b/docs/tint/style_guide.md
@@ -29,7 +29,7 @@
 
 ## Compiler support
 
-Tint requires C++14.
+Tint requires C++17.
 
 Tint uses the Chromium build system and will stay synchronized with that system.
 Compiler configurations beyond that baseline is on a best-effort basis.
@@ -45,6 +45,3 @@
 For test code, the tradeoff between readability and maintainability
 and other factors is weighted even more strongly toward readability
 and maintainability.
-
-
-